In traditional Japanese grammar, this is a 助動詞(jodōshi, “auxiliary verb”) with the defective paradigm たろ(taro), --, た(ta), た(ta), たら(tara), --. The 未然形(mizenkei, “irrealis or incomplete form”)たろ(taro) is used to derive the past conjectural suffix たろう(-tarō), which is now old-fashioned and taken over by だろう(darō) following the past form. The 仮定形(kateikei, “hypothetical stem”)たら(-tara-) used to further take the conditional particle ば(-ba), but now usually stands on its own to form the conditional.